High speed optical image acquisition system with extended dynamic range

ABSTRACT

A high-speed image acquisition system includes a source of light; a sensor for acquiring a plurality of images of the target; a system for determining relative movement between the source and the target; and an image processor for processing the acquired images to generate inspection information relative to the target. The system has an extended dynamic range provided by controlling illumination such that the plurality of images is acquired at two or more different illumination levels. In some embodiments, the high-speed image acquisition system is used to perform three dimensional phase profilometry inspection.

CROSS REFERENCE TO CO-PENDING APPLICATION

This application is a Continuation-in-Part application of U.S. patentapplication Ser. No. 09/522,519, filed Mar. 10, 2000 now U.S. Pat No.6,549,647, and entitled “Inspection System with Vibration ResistantVideo Capture,” which claims priority benefits from U.S. Provisionalpatent application Ser. No. 60/175,049, filed Jan. 7, 2000 and entitled“Improved Inspection Machine.”

COPYRIGHT RESERVATION

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

TECHNICAL FIELD

This invention relates to the acquisition and subsequent construction ofan extended dynamic range image using a high-speed imaging system.

BACKGROUND OF THE INVENTION

High-speed optical image acquisition systems are used in a variety ofenvironments to analyze the physical characteristics of one or moretargets. Generally, such systems include an image acquisition system,such as a camera, that can acquire one or more images of the target. Theimages are then analyzed to assess the target. In many cases, theinspection is performed while there is relative motion between thetarget and the image acquisition system. As used herein, a “high-speed”optical image acquisition system is any system that acquires images ofone or more targets while there is relative motion between the targetand an image acquisition system. One particular example of such a systemincludes a phase profilometry inspection system such as that used insome modern solder paste inspection systems.

Phase profilometry inspection systems are currently used to inspectthree-dimensional aspects of target surfaces. The concept of phaseprofilometry is relatively simple. A pattern or series of patterns ofstructured light are projected upon a target at an angle relative to thedirection of an observer. This can be likened to sunlight passingthrough a Venetian blind and falling upon a three-dimensional object,where the object is viewed from an angle that differs from that of thesunlight. The pattern is distorted as a function of the object's shape.Knowledge of the system geometry and analysis of the distorted image orimages can provide a map of the object in three dimensions.

Generally, phase profilometry systems employ a source of structured,patterned light, optics for directing the structured, patterned lightonto a three-dimensional object and a sensor for sensing an image ofthat light as it is scattered, reflected or otherwise modified by itsinteraction with the three-dimensional object.

Phase profilometry inspection can be performed while there is relativemotion between the target and the inspection system. This feature ishighly beneficial for automated inspection machines where throughput isvery important. One example of a phase profilometry inspection system isthe Model SE 300 Solder Paste Inspection System available fromCyberOptics Corporation of Golden Valley, Minn. This system uses phaseprofilometry to measure height profiles of solder paste deposited upon acircuit board prior to component placement. The SE 300 System is able toacquire the images that it uses for three-dimensional phase profilometrywhile allowing continuous relative motion between the sensor and thetarget. This is because a strobe illuminator is operated to provideshort exposure times thus essentially freezing motion. Specifics of theillumination and image acquisition are set forth in greater detail inthe parent application.

Solder paste, itself, presents a relatively favorable target in that itis comprised of a number of tiny solder spheres. Because there are somany small, spherical reflectors in each solder deposit, in theaggregate, each solder deposit provides a substantially diffuse opticalsurface that can be illuminated and imaged by a sensor configured toreceive diffusely scattered light. The relatively uniform reflectivityof the solder paste deposits facilitates imaging.

High-speed inspection systems such as the surface phase profilometryinspection described above provide highly useful inspection functionswithout sacrificing system throughput. Theoretically, such inspectionsystems would be highly useful for any inspection operation thatrequires height information as well as two-dimensional information froma system without adversely affecting system throughput. However, thereare real-world hurdles that hinder the ability to extend the advantagesof high-speed inspection beyond targets having relatively diffusereflectivity such as solder paste.

Surfaces such as the underside of a ball grid array (BGA) or chip scalepackages (CSP's) are difficult to inspect using current optical phaseprofilometry systems. Specifically, the balls on a BGA are constructedfrom reheated solder, rendering them substantially hemispherical andshiny. Illuminating the hemispherical shiny balls with a structured ordirectional light source will generate a bright glint from a portion ofthe surface near the specular angle. Conversely, there will be nearly nolight returned by diffuse scattering from the remainder of thehemispherical surface. Imaging the ball with a sensor that is notconfigured to deal with this wide dynamic range of returned light levelswould result in erroneous data.

When light falls upon any surface, some light will be specularlyreflected and some will be diffusely scattered. Some surfaces (like amirror) are predominantly specular in their reflections, and somesurfaces (like a piece of paper) predominantly scatter light. Imagingany such surface, even highly specular surfaces, may be possible byextending the dynamic range of a sensor configured to receive diffuselyscattered light. One way in which dynamic range has been extended in thepast is by taking or acquiring multiple images of a target withdifferent illumination levels and processing the images to discard imagedata such as saturated pixels or dark pixels. However, all dynamic rangeextension techniques currently known are believed to be applied solelyto systems in which the target and inspection system do not moverelative to each other while the multiple images are acquired. Thus, asdefined herein, such systems are not “high-speed.” In inspection systemswhere throughput cannot be sacrificed by pausing relative movementbetween the inspection system and the target, dynamic range extensionitself is not easy to implement.

SUMMARY OF THE INVENTION

A high-speed image acquisition system includes a source of light; asensor for acquiring a plurality of images of the target; a system fordetermining relative movement between the source and the target; and animage processor for processing the acquired images to generateinspection information relative to the target. The system has anextended dynamic range provided by controlling illumination such that aplurality of images is acquired at two or more different illuminationlevels. In some embodiments, the high-speed image acquisition system isused to perform three dimensional phase profilometry inspection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating operation of a high-speedimage acquisition system in accordance with an embodiment of the presentinvention.

FIG. 2 is a diagrammatic view of an inspection system in whichembodiments of the invention are particularly useful.

FIG. 3 is a flow diagram of a method of performing phase profilometryinspection in accordance with an embodiment of the present invention.

FIG. 4 is a flow diagram of another method of performing phaseprofilometry inspection in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION

Embodiments of the invention are applicable to any high-speed opticalimage acquisition system. While much of the disclosure is directed toinspection systems, particularly phase profilometry inspection systems,embodiments of the invention can be practiced in any image acquisitionsystem where image acquisition occurs without pausing relative motionbetween a target and image acquisition system.

In the phase profilometry embodiments, the number of images in eachimage set is preferably equal to the number of relevant unknowns in thetarget. For example, if the target has three substantial unknowns, whichmight include height, reflectivity and fringe contrast, then threeimages of three differing projected patterns or phases of a singlepattern or a suitable mix would be preferred. Thus, for embodiments withmore than three unknowns, more images would be preferred, and viceversa.

FIG. 1 is a diagrammatic view illustrating operation of a high-speedimage acquisition system in accordance with an embodiment of the presentinvention. A representative sensor 100 of a high-speed image acquisitionsystem is shown with light source 102. Source 102 is flashed, orotherwise energized for a very short duration one or more times, duringrelative movement between sensor 100 and target 104 as indicated byarrow 105. One or more images of target 104 is/are acquired by sensor100, corresponding to the flash(es), without pausing the relativemotion. After a short interval, inspection system 100 has moved to a newlocation indicated at 100 a in phantom. At the new location, whilemoving relative to target 104, illumination source 102 a is flashedagain, one or more times at an intensity that is different than thefirst flash or set of flashes. The relative flash intensities of thefirst and second sets of flashes are preferably measured, as will bedescribed later in the specification. Sensor 100 acquires image(s) oftarget 104 corresponding to the subsequent set of flashes. Sensor 100measures, or otherwise determines, the relative sensor movement betweenthe first and second flashes, or sets of flashes and uses thatinformation to register or cosite the two acquired image sets. Inembodiments where the number of images in each set is one, the detectorarray used in sensor 100 is preferably an interline transfer CCD arrayor CMOS array.

Once the image sets are registered, a composite image with extendeddynamic range can be created. Extended dynamic range is provided becauseif one image set has saturated or black portions, data from the otherimage can be used and normalized based on the measured illuminationintensities or the measured ratio between the two intensities.

Target 104 may be a two-dimensional or three-dimensional object.Further, the illumination generated during either or both flashes can beof any suitable wavelength, including that of the visible spectrum.

FIG. 2 is a diagrammatic view of an exemplary inspection system 200 inwhich embodiments of the present invention are useful. System 200includes image processor 202 and sensor 201. Sensor 201 includesprojector 203, and imager 206. Projector 203 includes illuminator 210,grating 212, and optics 214. Illuminator 210 is preferably a gasdischarge flashlamp, such as a high intensity xenon arc lamp, but can beany light source capable of providing short duration light pulses, suchas a pulsed laser or LED. Grating 212 imparts a pattern on the lightpassing therethrough, and optics 214 focuses the patterned light upontarget 216 on circuit board 218. Imager 206 includes optics 219, whichfocus the image of target 216 upon array 220, which is preferably aframe transfer CCD array because each set of images includes more thanone image in this embodiment. As the target moves in the Y directionrelative to sensor 201, the relative position of the target is observedby processor 202 via position encoders (not shown). Processor 202 iscoupled to projector 203 such that processor 202 triggers projector 203to project a patterned image upon target 216 on circuit board 218according to the observed position. Target 216 may be, for example, asolder paste deposit, an electrical component or a package of anelectrical component. As motion continues in the Y direction, two moreimages are acquired to complete the set of three images used in thisembodiment. The target motion relative to sensor 201 provides the phaseshift required to generate the p, p+120 and p+240 degree images used inthis embodiment to form a three-dimensional image of the target.

CCD array 220 provides data representative of the acquired images toimage processor 202 which computes a height map of target 216 based uponthe acquired images. The height map is useful for a number of inspectioncriteria.

FIG. 3 is a flow diagram of an embodiment of the invention whereextended dynamic range phase profilometry is performed using ahigh-speed inspection system. Method 300 begins at block 302, where afirst set of at least one image is acquired while the acquisition systemis moved relative to the target, and while the light source is flashedat a first illumination level. The first set of preferably three imagesis acquired in an extremely small amount of time in order to increasevibration immunity. For example, the first triplet can be obtained in 1millisecond or less, as disclosed in the parent application. Each imageof the first set is acquired while a different phase or pattern isprojected onto the target. Preferably, a sinusoidal fringe pattern isused, and each image is separated from the other by a lateral distancethat corresponds to approximately 120 degrees of phase change. Forexample, the images can be acquired at p, p+120, and p+240 degrees,where p is some arbitrary phase.

Typically, throughout inspection, there is continuous relative motionbetween sensor 201 and target 216. At block 306, a second set of atleast one image is obtained while target 216 is illuminated at a secondillumination level that differs from the first illumination level.Preferably, the second set consists of three images obtained withdiffering phases of a sinusoidal fringe pattern. The second set is alsoobtained within an interval of preferably one millisecond, but need notbe within any specific period from the acquisition of the first set, aslong as the difference is not so great that the two sets cannot berelated to one another. Preferably, however, if the first set of imagesis acquired at p, p+120, and p+240 degrees, the second set is acquiredat p+360n, p+360n+120 and p+360n+240 degrees, where n is an integer. Inthis manner, the images of the fringe pattern on target 216 will beidentical except for differences due to the intentional illuminationlevel change and the relative motion between sensor 201 and target 216corresponding to 360n degrees.

At block 308, the first and second sets of at least one image areregistered. This can be performed by adjusting one or both sets ofimages based upon measured, or otherwise known movement that occurredbetween acquisition of the first and second sets. Additionally, in someembodiments, it is possible to apply image processing techniques to thesets of images themselves to recognize patterns or references in theimages and manipulate the images such that the references are aligned.Further still, measured motion between the first and second sets ofacquisitions can be used to perform a first stage manipulation where oneor both sets are manipulated for a coarse alignment. Then, the resultsof the coarse alignment are processed to identify patterns or referencesin the coarse-aligned images in order to finely adjust the images. Oncethe images are registered, the dynamic range can be extended asdiscussed above.

Generally, the brighter set of images (those obtained using the greaterof the illumination levels) should be usable until saturation, presumingonly that the detector and analog circuitry are linear, or at leaststable (in which case non-linearity can be calibrated out). However, anysuitable technique or algorithm can be used.

Some embodiments of the present invention require that the difference inillumination levels be precisely controlled. Although it may betechnically possible to provide an illumination circuit with aprecisely-controlled intensity, it is preferred that the illuminationlevels be only approximately controlled, but then measured to therequired precision during use. As an example, the brighter illuminationlevel may be one hundred times brighter than the dimmer level. In oneembodiment, illumination levels are measured during use and used tocorrect for variations from one set to the next with sufficientprecision. Preferably, the required precision is approximately equal tothe smallest relative error detectable by the analog to digitalconverter. Thus, with a 10-bit converter, it would be sufficient tomeasure the relative light levels to a precision of one part in 2¹⁰;with a ratio of light levels of 100 to 1, the resulting dynamic rangewould be 100 times 2¹⁰ or about 100,000 to 1.

This approach is preferred due to the difficulty of controlling thestrobe discharge with the required precision.

Alternatively, each set of images can be constructed into height mapsbefore dynamic range extension and then three dimensional pixels(zetels) from the bright map can be replaced with zetels from the dimmap depending on whether there was saturation in the images used toconstruct the bright map. The strobe intensities must still be wellcontrolled or measured, since such intensity errors affect the heightreconstruction, but the task is easier since only the nominally-equalstrobe intensities within the set used to construct each height map needbe measured or controlled; the magnitude of the large ratio between setsneed not be known or controlled accurately.

Other normalization techniques are also possible. For instance, eachimage in the set can be normalized (that is, divided) by itscorresponding measured strobe intensity. Then the normalized pixels canbe freely chosen from one set or the other, based on their positions intheir respective dynamic ranges, with the height reconstruction donelater on the composite image set thus produced. This technique relies onthe fact that phase reconstruction is not affected by a scaling that isapplied uniformly to all the phase images. The suitability of aparticular technique may depend on the details of the hardware orsoftware used to make the selection.

At block 310, a height map is reconstructed using the composite set ofimage data from step 308. The manner in which the height map isgenerated can include any of the well-known phase profilometry heightreconstruction techniques. Once the height map is reconstructed, step312 is executed where profilometry inspection is performed on thereconstructed height map. This inspection can include extracting summarymeasures such as height, volume, area, registration, coplanarity,maximum height range, surface contours, surface roughness . . . etc.

FIG. 4 is a flow diagram of another method of performing phaseprofilometry inspection in accordance with another embodiment of thepresent invention. Method 400 begins similarly to method 300, with steps402, and 406 providing for the acquisition of first and second sets ofimages while the image acquisition system moves relative to a targetsurface. However, method 400 differs from method 300 in one importantrespect. Specifically, after step 406, method 400 provides step 408where height maps, one for each set of acquired images, arereconstructed. Then, at step 410, the height maps are reviewed todetermine if they include portions that were based on bad pixel data,such as saturated pixels. If a first height map includes such a portion,a corresponding portion of the second height map is reviewed todetermine if its pixel data for that portion is usable. If so, theportion of the first height map is replaced with the correspondingportion of the second height map. Since the maps are generated prior tostep 410, the replacement operation need not take into account thedifference in illumination intensity between the images used for thefirst height map and that of the second height map as long as thevariation of illumination intensity within a given set is the same, orcan be measured and corrected. At step 412, the final height map is usedin profilometry inspection. Although FIG. 4 illustrates acquiring thesecond set before performing any height construction, it is alsopossible to reconstruct the first height map before acquiring the secondset of images.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A high-speed image acquisition system comprising: a sensor movablerelative to a target; an illuminator adapted to direct illumination uponthe target at a plurality of illumination levels; wherein the sensorobtains a first set of at least one image at a first illumination levelduring relative motion between the sensor and the target, and the sensorobtains a second set of at least one image at a second illuminationlevel of the plurality of illumination levels during relative motionbetween the sensor and the target; wherein the first and secondillumination levels differ from each other by a known ratio, and whereinan unusable portion from one set of at least one image is replaced basedupon the other set of at least one image and the known ratio; and animage processor coupled to the sensor and adapted to create an extendeddynamic range image based at least in part upon cositing the first setof at least one image with the second set of at least one image.
 2. Thesystem of claim 1, wherein the illuminator is a stroboscopicilluminator.
 3. The system of claim 2, wherein the illuminator is a gasdischarge strobe.
 4. The system of claim 1, wherein the illuminator ismovable relative to the target.
 5. The system of claim 1, and furthercomprising a position measurement system coupled to the sensor, theposition measurement system providing an indication to the imageprocessor relative to displacement occurring between acquisition of thefirst and second sets.
 6. The system of claim 5, wherein the imageprocessor registers the first and second sets based upon the indication.7. The system of claim 6, wherein the image processor registers thefirst and second sets based upon image processing of the first andsecond sets.
 8. The system of claim 1, wherein the target is anelectrical component.
 9. The system of claim 1, wherein the target is apackage of an electrical component.
 10. A high-speed inspection systemcomprising: a single source disposed to project light upon a target at aplurality of illumination levels; an imaging system for acquiring aplurality of sets of at least one image, wherein a first set is relatedto a first illumination level of the plurality of illumination levels,and a second set is related to a second illumination level of theplurality of illumination levels, during relative movement between theimaging system and the target; a processor for processing the acquiredimages to generate inspection information with an extended dynamicrange; and wherein the source is adapted to project patternedillumination, wherein the pattern is a sinusoidal fringe pattern, andwherein the first set of at least one image includes three imagesacquired at phases that are substantially P, P+120, and P+240 degrees ofthe sinusoidal function.
 11. The system of claim 10, wherein the firstset is acquired within one millisecond.
 12. The system of claim 10,wherein the second set of at least one image includes three imagesacquired at phases that are substantially p+360n, p+360n+120, andp+360n+240 degrees of the sinusoidal function, where n is an integer.13. The system of claim 12, wherein the second set is acquired withinone millisecond.
 14. The system of claim 10, wherein the source is astrobe.
 15. The system of claim 14, wherein the source includes a gasdischarge flashlamp.
 16. The system of claim 15, wherein the source is axenon arc flashlamp.
 17. The system of claim 10, wherein the first andsecond sets of images are registered using image processing techniques.18. The system of claim 10, and further comprising a system formeasuring displacement between the imaging system and the target duringthe interval between acquisition of the first and second sets, andwherein the first and second sets are registered based at least in partupon the measured displacement.
 19. The system of claim 10, and furthercomprising an illumination sensor for sensing the first and secondillumination levels to provide a ratio therebetween.
 20. The system ofclaim 19, wherein the processor registers the first and second sets tocreate a composite set.
 21. The system of claim 19, wherein portions ofimages in one set are replaced by portions of images in the other set bynormalizing the portions in the other set using the ratio.
 22. Thesystem of claim 10, and further comprising an illumination sensor forsensing the first and second illumination levels to provide a differencetherebetween.
 23. The system of claim 22, wherein the processorregisters the first and second sets to create a composite set.
 24. Thesystem of claim 22, wherein portions of images in one set are replacedby portions of images in the other set by normalizing the portions inthe other set using the ratio.
 25. A method of inspecting a target witha high-speed inspection system, the method comprising: obtaining a firstimage of the target at a first illumination level with a sensor whilethe sensor and target move relative to each other; obtaining a secondimage of the target at a second illumination level with the sensor whilethe sensor and target move relative to each other, wherein the first andsecond illumination levels differ from each other by a known amount;replacing an unusable portion of one of the first and second images witha corresponding portion of the other of the first and second images bynormalizing the corresponding portion based upon the known amount ofdifference in illumination levels, to create a composite image; andprocessing the composite image to generate inspection informationrelative to the target.